Where layers of fat should have covered whales' spines last winter, researchers saw vertebrae sticking out. They spotted other signs of malnutrition - depressions around the blowholes and head, and protruding shoulder blades - that may indicate declining health.

At least 10 percent of gray whales returning to Laguna San Ignacio, one of four main calving and breeding lagoons off Baja California, Mexico, showed signs of being underfed, said Steve Swartz, a National Marine Fisheries Service whale expert based in Silver Spring, Md.

Researchers are trying to find out if it's a warning sign that climate change in the North Pacific is affecting the tiny crustaceans the whales suck up from the ocean floor, and if switching to alternative prey will affect their well-being.

"They may have to work harder for less," Swartz said. "There are all kinds of things we're thinking about, trying to piece the puzzle together."

The eastern population of Pacific gray whales are a marine mammal success story. Nearly wiped out by hunting by the early 1900s, they're the only marine mammal to be delisted from Endangered Species Act protections.

They breed in warm, protected lagoons off Mexico's coast but rely on the cold, bountiful waters off Alaska for feeding each summer.

However, traditional feeding areas in the Bering and Chukchi seas have been disrupted by ecosystem changes associated with Arctic warming. Swartz said that's a possible reason for physical changes observed last winter. It may also be a factor, Swartz said, in a decline of calf production.

Other whale experts are not sure there's anything out of the ordinary.

Wayne Perryman, who oversees the annual gray whale census for NOAA Fisheries, said observations by veteran researchers in the breeding lagoons cannot be discounted. However, "There's no way of knowing if what they saw was a representative sample of the population," Perryman said.

It's natural for whales to be thin, especially for lactating females, because they fast for months at a time on their journeys. And just because they're slim, it doesn't mean their health is at risk, he said.

"There's kind of a natural range of skinny," he said.

But like other gray whale researchers, he believes the observations are worth following up. As an ocean sentinel reflecting the health of the Pacific Ocean, gray whales are uniquely qualified, he said.

Their migrations may take them on a 10,000-mile round trip through long stretches of the ocean. Some pass through the Bering Strait into the Arctic Ocean. A one-way trip takes two to three months.

On both legs of the trip past California, they pass close to shore, making it relatively inexpensive for scientists such as Perryman to count them. That's in contrast to polar bears roaming on top of remote Arctic waters, or their primary prey, ringed seals, which spend much of their lives hidden from view in snow lairs on top of Arctic sea ice.

Gray whales feed on small creatures such as amphipods, a small, shrimp-like crustacean, and tube worms found in bottom sediments. The whales have 130 to 180 overlapping plates of frayed baleen hanging from each side of the upper jaw. When they feed, gray whales dive to the ocean bottom, roll on their side and draws bottom sediments and water into their mouth. As they close their mouths, they expel water and sediment through the baleen plates, which trap the food on the inside near the tongue to be swallowed.

Perryman would be more worried about the health of the gray whale population, he said, if the observations of skinny whales were accompanied by strandings, a term scientists use not just to describe whales that become disoriented and end up beached, but also whales that die of starvation, disease or parasites and float to shore.

The whale population reached a modern peak of 26,000 animals in 1997. But in 1999, the federal government recorded about 270 strandings, and a year later, more than 300. Calf production fell and the population dropped to 19,000 animals.

Carl Safina, president of Blue Ocean Institute, a conservation organization, noted research that shows how diminished sea ice may be forcing gray whales to swim hundreds of miles farther north to find food.

Swartz hopes there will not be another crash in the gray whale population.

He also worries about the effects on species not as easily counted as gray whales.

"We would expect by supposition, by assumption, that there would be a similar effect," he said. "It would be interesting to see how they're responding."